AD1893 데이터 시트보기 (PDF) - Analog Devices

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AD1893

Low Cost SamplePort® 16-Bit Stereo Asynchronous Sample Rate Converter

Analog Devices 

AD1893

Polyphase Filter Bank Model

Although less intuitively understandable than the interpolation/

decimation model, the polyphase filter bank model is useful to

explore because it more accurately portrays the operation of the

actual AD1893 SamplePort hardware. In the polyphase filter

bank model, the stored FIR filter coefficients are thought of as

the impulse response of a highly oversampled 0 kHz to 20 kHz

low-pass prototype filter, as shown in Figure 2. If this low-pass

filter is oversampled by a factor of N, then it can be conceptu-

ally decomposed into N different “subfilters,” each filter consist-

ing of a different subset of the original set of impulse response

samples. If the temporal position of each of the subfilters is

maintained, then they can be summed to recreate the original

oversampled impulse response. Since the original impulse re-

sponse is highly oversampled, the more sparsely sampled

subfilters still individually meet the Nyquist criterion (i.e., they

are adequately sampled). The baseband magnitude and phase

responses of the subfilters are identical. The out-of-band (i.e.,

alias) regions of the subfilters however have phase responses

which are shifted relative to one another, in a manner that

causes them to cancel when they are summed.

The subfilter coefficients are then aligned to the left, as shown

in Figure 3, so that the first coefficient of each subfilter is

aligned to the first point on a coarse time scale. (This conceptual

step accounts for how the hardware implementation is able to

operate at the slower rate corresponding to the coarse time

scale.) Each subfilter has been shifted in time by a different

amount, and though they still share identical magnitude re-

sponses, they now have in-band phase responses which have

fractionally different slopes (i.e., group delays).

AMP

OVERSAMPLED

LOW-PASS FILTER

IMPULSE RESPONSE

AMP

TIME

PHASE

0 Deg

270

90

180

1/4FS

1/2FS

3/4FS

FREQ

FS

DECOMPOSED INTO

FOUR SUBFILTERS

1/4FS

1/2FS

3/4FS

FS

1/4FS

1/2FS

3/4FS

FS

1/4FS

1/2FS

3/4FS

FS

1/4FS

1/2FS

3/4FS

FS

Figure 2. Four Polyphase Subfilters in the Time and Frequency Domains

–8–

REV. A

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